There's and yin and yang to marathon training. It's impossible to race 26.2 miles without some fairly rigorous training (unless you want to be in a world of pain!!).
At the same time, training too hard by increasing mileage too quickly or simply running too fast without adequate recovery has its drawbacks. Obviously, completing an assortment of workout builds up muscles and blood vessels and heart and lung and breathing stuff. More ambiguously, training boosts your confidence so you can tackle a tougher race pace and muster the attitude kick it in at the end. If workouts are tough or if I'm sweating a lot, I remind myself that it'll make the race seem like cake :P A 9:40 pace would have me finishing the SF marathon around 4:15, but I have some confidence that I can average 9:30 miles...Yikes! Typing that made me feel less confident and more ouchy.
Today's medium long run felt great and raised my confidence that I'm able to cruise along at a solid pace for longer distances. Workouts like today make me realize that I'm getting stronger because when you're stronger, you feel great cruising down the road- well, sidewalk.